Zirconium metal has historically been used primarily as an internal material of construction for nuclear reactors, for instance, as claddings for uranium oxide nuclear fuel rods. Cladding of nuclear fuel rods, the primary end, provides an outer metallic jacket generally surrounding the nuclear fuel element which serves, inter alia., to prevent corrosion of the fissionable nuclear fuel and release of fission products into the coolant loop. Other attractive applications for zirconium metal are in the fabrication of corrosion-resistant chemical process hardware and advanced ceramics as oxides.
Particularly, materials for use in nuclear reactors are selected for their thermal neutron capture cross-sections, along with other properties. Zirconium is selected in nuclear application for, among other properties, its low, average thermal neutron capture cross-section (approx. 0.18 barns), good ductility, good resistance to radiation damage, and excellent corrosion resistance in pressurized hot water of temperatures up to about 350.degree. C. The naturally occurring isotopic distribution of zirconium has a low average thermal neutron capture cross-section which is desirable for certain nuclear reactor materials. The naturally occurring isotopes of zirconium are given in Table 1.
TABLE 1 ______________________________________ Naturally Occurring Zirconium Isotopes Thermal Neutron Capture Isotope Occurrence, % Cross-Section, Barns (10.sup.-28 m.sup.2) ______________________________________ Zr.sup.90 51.45 0.03 Zr.sup.91 11.32 1.14 Zr.sup.92 17.49 0.21 Zr.sup.94 17.28 0.055 Zr.sup.96 2.76 0.020 ______________________________________
However, there has been a continuing interest in reducing the tendency of nuclear grade zirconium to absorb thermal neutrons due to its contamination by neutron opaque hafnium. The more transparent the internal materials of construction of a nuclear reactor are to these thermal neutrons, the more efficiently the nuclear reactor will function since a certain number of these thermal neutrons are required to sustain the neutron triggered fission reactions. Accordingly, it is desirable to reduce the thermal neutron capture cross-section of the internal materials of construction of a nuclear reactor, such as zirconium.
Early efforts at reducing the thermal neutron capture cross-section of zirconium were directed to separating zirconium from hafnium, which by the way are the most difficult elements of the periodic table to separate. The two elements, zirconium and hafnium, occur together naturally, but hafnium has a substantially larger thermal neutron capture cross-section (almost 600 times that of zirconium). Thus the separation of zirconium from hafnium allows for the production of nuclear grade zirconium with a lower average thermal neutron capture cross-section by the elimination of hafnium.
Commercial processes currently available for the production of nuclear grade zirconium are variations of solvent extraction processes wherein zircon sand is converted to zirconium metal as a result of a somewhat involved series of steps. This extraction process requires the use of organic solvents, usually hexone, and various aqueous solutions, including hydrochloric acid. Hafnium, which is chemically similar to zirconium, must be separated from the zirconium. Usually a hexone/thiocyanate/hydrochloric acid system is employed for this purpose and requires a series of separate extraction steps along with separate separation columns. The zirconium, organic solvent and thiocyanate recovered from the hafnium separation steps are usually subject to additional processing to make sure that as much zirconium is recovered from the system as possible. The zirconium ultimately recovered from most extraction processes is in the form of pure zirconium oxide (ZrO.sub.2). In the commonly used commercial process, the zirconium oxide is then chlorinated to form ZrCl.sub.4, which is purified and subjected to Kroll reduction to produce zirconium metal suitable for nuclear applications. The aqueous and organic liquids used in the process typically include waste metals and other materials that must be properly discarded. One of the methods of treating these liquid wastes is to place them in holding ponds for future treatment and remediation. However, this is increasingly becoming an unacceptable waste management solution, particularly since federal and state laws relating to waste disposal have become more stringent.
These solvent extraction processes do effectively separate zirconium from hafnium in order to produce zirconium of the quality required for use in nuclear reactors and elsewhere in the nuclear industry. However, the increasing concern expressed by the public, the scientific community and the regulatory agencies regarding the waste generated by solvent extraction processes has lead the nuclear industry to explore alternative zirconium production methods which do not present the same waste management concerns.
Other zirconium and hafnium separation processes in addition to the aforementioned solvent extraction processes have been proposed. Recent efforts have proposed economically practical techniques of separation of zirconium from hafnium and also for separation of zirconium and hafnium isotopes therefrom by using continuous, steady-state, ion exchange chromatography in a continuous annular chromatograph (CAC) as taught in, for example, U.S. Pat. No. 5,023,061 (Snyder, et al.), U.S. Pat. No. 5,024,749 (Snyder, et al.), U.S. Pat. No. 5,098,676 (Lee, et al.), U.S. Pat. No. 5,110,566 (Snyder, et al.), U.S. Pat. No. 5,112,493 (Snyder, et al.), and U.S. Pat. No. 5,174,971 (Snyder, et al.). These patents all teach continuous methods and systems for simultaneously separating and purifying zirconium from hafnium and/or simultaneously isolating the fairly abundant low thermal neutron capture cross-section isotopes of zirconium and the high thermal neutron capture cross-section isotopes of hafnium in a single operation. While each of these teachings are technically feasible, optimization of the process for greater efficiency, lower waste generation and lower costs cannot be adduced therefrom. What is needed is a method and system for chromatographically separating zirconium from hafnium in a single operation using optimized processing parameters which provide for commercially important operations.
In the past, commercially acceptable zirconium and hafnium separations in a CAC device supported by using crude aqueous chloride feedstock solutions derived from the aqueous hydrolysis of the carbochlorination products of zircon sand, and by using aqueous chloride or sulfate eluant solutions along with cationic or anionic exchange resins have proved to be elusive. Zirconium product purity of less than 50 ppm [Hf]/[Zr], zirconium product concentrations of greater than 15 g/l [Zr], and product yields of about 90% have been unobtainable using crude aqueous chloride feedstock solutions in the CAC. What is need is an improved method and system for continuously separating and purifying zirconium and hafnium and also, if desired, for continuously and selectively enriching zirconium and hafnium isotopes, in order to produce nuclear grade materials by using continuously operating, steady-state, ion exchange chromatographic techniques and by using improved feedstock solutions and eluant solutions and improved operating conditions in a CAC device which provide substantially improved product purities, concentrations, and yields, while also eliminating both liquid waste discharge and organic reagents. Accordingly, the present invention addresses design improvements in order to improve the overall quality of the resultant nuclear grade materials in terms of: greater purification efficiencies, greater yields, simpler process design, lower operating costs, lower waste generation, and others.